1. Field of the Invention
The present invention relates to a method of connecting plastic tubes by high-frequency heating, and more particularly to a method of connecting plastic tubes by high-frequency heating so as to ensure a long, continuous conduit. The conduits are particularly adapted for medical applications where no hygienic problems are permitted.
2. Description of the Prior Art
To effect the connection between flexible plastic tubes or pipes such as those of polyvinyl chloride, there are at least three methods; one is a method of using an adhesive, another is a method of utilizing the fusible nature of the plastic tubes, commonly called the "thermal sealing method", and the third one is a method of high-frequency heat. The adhesive method is self-explanatory. The thermal sealing method consists of fusing the tube ends to be joined under pressure so as to ensure the joint therebetween. A typical example of the high-frequency heating method is disclosed in Japanese Laid-Open Patent Publication No. 59-178214. This high-frequency heating method consists of abutting tube ends to be joined within a cylindrical outer electrode with an inner electrode being inserted in the tubes, and then applying a high frequency voltage between the inner and outer electrodes. The tube ends fuse and become joined. There is another method of combining the adhesive method and the fusion joint method.
A disadvantage of the adhesive method is that organic solvents such as tetrahydrofuran and cyclohexanon contained in the adhesive are likely to remain in the joined tube ends and harm patients' health when the tubes are used for medical treatments such as flood transfusion and catheters. A transfusion bag is legally prohibited from containing an adhesive. For medical applications the adhesive method must be avoided because of hygienic considerations.
A disadvantage of the heat sealing method is that the surface of flexible tube are likely to become carbonized before it reach a fusible temperature. This makes the tubes difficult to use in catheters. In addition, the joint strength is not sufficient.
The high-frequency heating method described above has the following disadvantages:
One is a hygienic problem. As described above, the inner electrode is inserted into the tubes, which is likely to introduce germs and/or dirt into the tubes and unhygienically contaminate the insides thereof. Another disadvantage is that the inner electrode must be manually inserted, which consumes labor and time. A further disadvantage is that the tube ends may contract and tightly constrict around the surface of the inner electrode to such an extent that the interfacial adhesion prevents the inner electrode from being pulled out of the tubes. This also consumes time and labor before the electrode is withdrawn from the tubes. A still further disadvantage is that this method can be only applicable when the tubes are open at both ends. If the tubes are closed at one end, the inner electrode would be confined in the conduit and could not be extracted therefrom. The applicability of the method is limited.
There is another high-frequency heating method, which is disclosed in Japanese Laid-Open Patent Publication No. 59-232820. This prior method is designed to join plastic plates. According to this method, plastic plates are overlaid in their end portions, and a pair of electrodes are placed on one of the plates, most often on the upper plate. An insulator is disposed between the two electrodes. A high-frequency voltage is applied between the electrodes. If this method is applicable to the joining of tubes, it would be advantageous in (1) that an unhygienic condition is unlikely to occur, (2) that the bother of inserting an inner electrode into the tubes is eliminated, and (3) that the method can be applied to any type of tube whether they may be closed at both ends or not. However, when tubes are to be joined by this method, it is unavoidable that the tubes be rotated so as to ensure the peripheral fusion joint. The rotation of tubes requires the provision of a rotor, which increases the production cost and prolongs the operation time. Nevertheless, it is likely that the joint would be incomplete.